Hello, we had similar problem. Current jdbcio will cause alot of connection errors.

Typically you have more than one prepared statement. Jdbcio will create for each prepared statement new connection(and close only in teardown) So it is possible that connection will get timeot or in case in case of auto scaling you will get to many connections to sql.

Our solution was to create connection pool in setup and get connection and return back to pool in processElement.

We are working on a flow to ingest events and writes the aggregated
counts to a database. The input rate is rather low (~2000 message per
sec), but the processing is relatively heavy, that we need to scale out
to 5~6 nodes. The output (via JDBC) is aggregated, so the volume is also
low. But because of the number of workers, it keeps 3000 connections to
the database and it keeps hitting the database connection limits.

Is there a way that we can reduce the concurrency only at the output
stage? (In Spark we would have done a repartition/coalesce).

And, if it matters, we are using Apache Beam 2.2 via Scio, on Google
Dataflow.